Conventional Schottky rectifiers are used in high speed application as alternatives to the classical PIN diodes. They have limited blocking range and their primary success is in applications that require breakdown voltages below about 200V. The main reason for their limited range of blocking voltages is due to the severe increase in the on-state forward voltage drop at high breakdowns, which in turn is caused by the decrease in the doping concentration of the drift region and concomitantly by an increase in the depth of the drift region. As a result, the specific on-state resistance of the drift region is approximately proportional to VBR2.5, where VBR is the breakdown voltage. This superlinear relationship between the on-state resistance and the breakdown voltage makes it difficult for Schottky rectifiers to address the market for higher blocking voltages. In addition, the high electric field present in the Schottky contact leads to a barrier lowering effect and therefore high leakage currents at high blocking voltages.
FIGS. 1 and 2 show a conventional PIN diode and a conventional Schottky rectifier, respectively. The PIN diode includes a highly doped semiconductor substrate 110 that is heavily doped with a dopant of a first conductivity type (e.g., n+ type). An epitaxial drift layer 120 is formed on the substrate 110 and is more lightly doped with a dopant of the first conductivity type (e.g., n−type). A heavily doped ohmic contact layer 130 is formed on the drift layer 120. The contact layer 130 is heavily doped with a dopant of the second conductivity type (e.g., p+ type). A cathode electrode 150 is formed on the backside of the substrate 110 and an anode metal 140 is formed over the ohmic contact layer 130.
The conventional Schottky rectifier shown in FIG. 2 includes a highly doped substrate 210 that is heavily doped with a dopant of a first conductivity type (e.g., n+ type). Similar to the PIN diode, a drift layer 220 is formed on the substrate 210 and is more lightly doped with a dopant of the first conductivity type (e.g., n− type). Then, instead of an ohmic contact layer, a metal layer 230 is formed over the drift layer 230. A Schottky contact is formed at the interface between the metal layer 230 and the drift layer 220. A cathode electrode 250 is formed on the backside of the substrate 210 and an anode metal 240 is formed over the metal layer 230.
To reduce the susceptibility of the Schottky contact to the electric field, a Trench MOS Barrier Schottky (TMBS) device has been developed. This device features multiple trench MOS cells in its active region to reduce the surface electric field and create a lateral barrier which opposes the flow of the leakage currents. As a result the off-state leakage currents are significantly reduced. Moreover, the MOS trenches also act as field plates and thus allow for a slight increase in the doping of the drift region without compromising the breakdown. However, the on-state voltage drop of the TMBS device remains an issue in high voltage applications, where the breakdown is in excess of 300V. This is because the unipolar conduction mechanism of the Schottky rectifier is not as effective as the bipolar conduction specific to PIN type diodes.